Metal mold for injection molding

ABSTRACT

A metal mold for the injection molding, characterized in that a plastic piece being lower in thermal conductivity than the base metal of said metal mold is provided to form at least a part of the inside of said metal mold, and that a metal plating layer is deposited on the exposed surface of said plastic piece.

United States Patent 1 Ron et a1.

METAL MOLD FOR INJECTION MOLDING Inventors: Moriyuki ltou, lwatsuki-shi,

Saitama-ken; Takashi Aoba, Ageoshi, Saitama-ken, both of Japan Assignee:Tokyo Shibaura Electric Co., Ltd.,

Kawasaki-shi, Japan Filed: Aug. 6, 1971 Appl. No.2 169,785

Foreign Application Priority Data Oct. 14, 1970 Japan ..45/l0l398 US.Cl. ..249/ll4, 249/13 4, 425/470 Int. Cl ..B29c 1/02 Field of Search...425/l17, 470; 249/116, 135, 114, 134; 264/338; 164/123 51 May 22,1973 [56] References Cited UNITED STATES PATENTS 2,390,373 12/1945 Joneset al ..249/201 3,072,981 1/1963 Davids0n....... 3,204,917 9/1965Richards ..249/1 16 Primary ExaminerR. Spencer Annear Attorney-Xenon,Palmer & Estabrook ABSTRACT A metal mold for the injection molding,characterized in that a plastic piece being lower in thermalconductivity than the base metal of said metal mold is provided toformat least a part of the inside of said metal mold, and that a metalplating layer is deposited on the exposed surface of said plastic piece.

2 Claims, 1 Drawing Figure METAL MOLD FOR INJECTION MOLDING The presentinvention relates to a metal mold for injection molding, and moreparticularly to a metal mold which imparts luster to the surface of aninjectionmolded article of a thermoplastic resin.

In the injection molding of thermoplastic resins such as polystyrene andpolypropylene, it is known that the luster of a molded article variesdependent upon the temperature of the metal mold. As the temperature ofthe metal mold is higher, the luster will become better.

In case where a molded article of a low-expansion foaming resin ismolded by the injection molding, when a substance of a lower thermalconductivity in comparison with that of the base metal of the metal moldis interposed at the inner surface of the metal mold, that surface ofthe molded article which corresponds to the substance portion exhibits adecrease in the appearance of a whitish form pattern and is coloredsomewhat closer to the ground or base color of the resin to bedistinguished from the foamed pattern other than it.

It is impossible, however, to make the surface have substantially thesame color as the ground color of the resin and to impart a sufficientluster thereto.

An object of the present invention is to provide a metal mold forinjection molding, in which the inside thereof is held at a highertemperature locally or over the entire area during molding.

Another object of the present invention is to provide a metal mold whichimparts luster to part or the whole of the surface of aninjection-molded article consisting of usual thermoplastic resins.

Still another object of the present invention is to provide a metal moldwhich causes a lustrous ground color of a resin to appear on part or thewhole of the surface of an injection-molded article consisting oflowexpansion foaming thermoplastic resins.

More specifically, the present invention is constructed such that, atthe inside of a metal mold for the injection molding, a layer of asubstance lower in thermal conductivity than the base metal of the metalmold, i.e., of a resin, is depositionformed at least at part of theinside of the metal mold, that a thin metal plating layer is provided onthe resin layer to thus form a composite layer, and that the surface ofthe composite layer is made a metal-mold surface, i.e., a surface atwhich the metal mold and a material resin are held in contact. When thematerial resin in the molten state flows into the metal mold, the metalplating layer is rapidly heated under the action of heat insulation ofthe layer of the substance having a lower thermal conductivity, totemporarily raise the temperature of the inside of the metal mold,thereby more enhancing the luster of a molded article.

The present invention can be more fully understood from the followingdetailed description when taken in conjunction with reference to theappended drawing, in which:

The single FIGURE is a longitudinal section which shows a metal moldembodying the present invention.

When a metal mold of the present invention is applied to the molding ofa molded article of a lowexpansion foaming resin, that surface of themolded article which corresponds to a composite layer consisting of alayer of a substance of a lower thermal conductivity and a plated layerpresents a color phase extremely close to the ground or base color ofthe resin, and therewith, it becomes a lustrous surface. Accordingly,

in the case where the entire inner surface of the metal mold is formedinto the composite layer of the present invention, even if alow-expansion foaming resin is employed, there may be obtained a moldedarticle whose surface has an external appearance substantially the sameas in the use of usual resins and exhibits luster while its inner partis of a foamed structure.

According to the metal mold of the present invention, it is presumed, atthe injection molding, the metal plating layer is temporarily broughtinto the heatinsulated condition and is maintained at a hightemperature, and thus the smooth surface thereof adheres sufficientlyclosely to a molten material resin, so that the luster of thecorresponding surface of the injectionmolded article may be improved.While conditions for attaining a good luster vary dependent upon theresin employed and are controlled by the thickness of the resin layer ofa lower thermal conductivity, they may be determined by experiments soas to accomplish the best results without substantially lengthening themolding time.

For the resin smaller in the thermal conductivity relative to the basematerial of the metal mold as is used in the present invention, theremay be used an epoxy resin, a polyethylene terephthalate resin, anacrylonitrile-butadienestyrol resin, a phenol-formaldehyde resin, andother resins. The thickness of the resin layer is selected atapproximately 0.3 to 1 mm. For the metal to be plated on the resinlayer, there may be used nickel, chromium, cadmium, gold, silver, tin,and the like. In case where it is difficult to plate the metals directlyon the resin layer, a rigid plating layer is attainable throughinterposition of a plating layer of other metals.

In the accompanying drawing, a metal mold 1 consists of a female mold orcavity 2 and a male mold or core 3 engaged therewith. Between the cavity2 and the core 3, a molding void portion 4 is defined. The cavity 2 isprovided with a sprue 5 through which a molten material resin issupplied at molding, while the cavity 2 and the core 3 are respectivelyformed with a plurality of circulation holes 6 and 7 for a coolingmedium. The present invention disposes, on the inside of such metalmold, a composite layer which consists of a layer of a substance lowerin thermal conductivity than the base metal of the metal mold and aplated layer. In the illustrated embodiment, the composite layerconsisting of the above-mentioned substance layer 8 and the metalplating layer 9 is provided at part of the inside of the molding voidportion of the cavity 2, while the composite layer consisting of thesubstance layer 10 and the metal plating layer 11 is disposed on theentire area of the inside of the molding void portion of the core 3. Inorder to provide such composite layers, in case of the cavity 2, aconcave place 12 is perforated in the inside thereof, the layer 8 of,e.g., epoxy resin is deposited on the bottom surface of the concaveplace 12, and'then, the metal plating layer 9 is formed on theepoxy-resin layer 8. On the other hand, in case of the core 3, theinside is formed into a surface 13 which is lower than the completedinner surface by a thickness of the composite layer consisting of theresinous layer 10 and the metal plating layer 11. In case where theresinous layer is thin, said resinous layer and the plated layer may beformed on the lower-level surface 13. In case where the resinous layeris' relatively thick, a separate cavity as forms between it and thelower-level surface 13 a void portion corresponding to the resinouslayer is previously prepared, the epoxy resin, by way of example, may befilled into the void portion to deposit the resinous layer, and then,the plated layer may be formed. Thus, in the illustrated embodiment, asurface 14 of the composite layer of the cavity 2 constitutes part ofthe cavity surface, while a surface 15 of the composite layer of thecore 3 provides the whole core surface.

The depths of the concave place and the lower-level surface provided atthe inside of the metal mold of the present invention may be, inactuality, selected equal to the respective thicknesses of the resinouslayers only, since the metal plating layers are thin. In case where theresinous layer is disposed at part of the inside of the metal mold, themetal plating layer may be provided not only on the resinous layer, butalso commonly on other areas of the metal-mold surface: withouthindrance. With such metal plating layer, the surface of the metal moldmay be advantageously protected from corrosion and abrasion.

Description will now be made of actual examples of the new molds thepresent invention and their uses.

EXAMPLE 1 In a metal mold made of steel which consisted of female andmale molds having a void portion of 80 mm in diameter and 3 mm in depth,a concave portion of 10 mm in diameter and 0.3 mm in depth wasperforated in the circular bottom surface of the female mold or cavity,and an epoxy-resin film approximately 0.3 mm thick was caused to adhereinto the concave portion to cover it.

The covering surface of the epoxy-resin film was repeatedly washed withan aqueous solution of caustic soda, water, hydrochloric acid, andwater. Thereafter, using CUPPEY MIXS SOLUTION (tradename) which is acopper electrolessplating solution produced by Shipley Co. andcontaining an aqueous solution of cop per sulfate as its mainconstituent, a copper electrolessplating layer 10 p. thick was formed.Then, using an aqueous solution having the composition of 210 g/l ofcopper sulfate and 60 g/l of sulfuric acid and under the conditions of acurrent density of 2 AmpJdm a specific gravity of 1.2 and a temperatureof the solution of 50C, a copper electroplating layer 0.01 mm thick wasdeposited on the said electrolessplating layer.

A nickel electroplating layer having a thickness of 0.01 mm was formedonto the copper electroplating layer, using a nickel-plating aqueoussolution having the composition of 300 g/l of nickel sulfate, 50 g/l ofnickel chloride, 50 g/l of boric acid and 0.5 g/l of gelatin and underthe conditions of a current density of 3 AmpJdm, pH 5.0 and atemperature of the solution of 40 to 50C.

This metal mold was pre-heated to 40C, and with a styrol resin which isknown under the tradename of DIAREX PIT-88A (produced byMitsubishi-Monsanto Chemical Co., Ltd.), the injection molding wascarried out at a cylinder temperature of 200C and under an injectionpressure of 1,000 kg/cm. One side of a molded disc obtained, presented avery lustrous external appearance.

EXAMPLE 2 In a metal mold made of steel which consisted of a cavity anda core defining a void portion of 80 mm in diameter and 3 mm in depth, aconcave portion of 10 mm in diameter and 0.5 mm in depth was carved intopart of the inside of the core, and an epoxy resin was filled anddeposited into the concave portion. Over the entire area of the coreinside including the deposited epoxy-resin layer, there weresuccessively formed a copper electrolessplating layer 0.008 mm thick, acopper electroplating layer 0.01 mm thick and a nickel electroplatinglayer 0.01 mm thick in the treating order mentioned in Example 1.Thereafter, using an aqueous solution of the composition of 200 g/l ofchromium sulfate, 240 g/l of urea and 396 g/l of ammonium sulfate andunder the conditions of a current density of 35 Amp/din, pH 2.5 and atemperature of the solution of 45C, treatment was done to form a chromeelectroplating layer 0.02 mm thick on the nickel plating layer.

Using this metal mold, the injection molding was carried out with thestyrol resin mentioned in Example 1 and in conformity with the processin Example 1. A lustrous, small circle was clearly observed on one sideof a molded disc obtained.

EXAMPLE 3 Herein explained is a case where, in the metal mold of Example2, on the nickel electroplating layer there was disposed a cadmiumplating layer instead of the chrome plating layer.

Treatment was done using an aqueous solution of the composition of 25g/l of cadmium sulfate, 130 g/l of sodium cyanide and 35 g/l of causticsoda and under the conditions of a current density of 1.5 Ampjdm avoltage of 10 V and a temperature of the solution of 25C, and a cadmiumelectroplating layer 0.02 mm thick was formed on the nickel platinglayer.

With the above metal mold, the injection molding was performed inaccordance with the process in Example l by applying a low-magnificationfoaming plastic material which was made up such that, to a styrol resinwith DlAREX l-lT-88A and DIAREX HF-77 (both being tradenames and beingproduced by Mitsubishi-Monsanto Chemical Co., Ltd.) compunded by equalparts, 1 percent of diazo-carbon amide as a foaming agent and 3 percentof Dai-Nippon-Ink-Dairen Color SP 705 (tradename, produced by Dal-NipponInk and Chemicals, Inc.) as a pigment were respectively mixed. On oneside of a molded disc obtained, a small circle of a lustrous groundcolor of the resin appeared as part of a whitish foamed pattern, and wasclearly distinguishable.

EXAMPLE 4 In a metal mold made of a beryllium-copper alloy which had avoid portion of mm in diameter and 3 mm in depth, a heart-shaped concaveportion being approximately 10 mm in both height and width and being 3mm in depth was perforated in a part of the inside of a cavity. A moldedarticle of an acrylonitrilebutadiene-styrol resin which had aconfiguration conforming to the above shape was made, and it was fittedto the concave portion and was secured with an epoxyresin bonding agent.The surface of the resinous covering was treated by the process statedin Example 1, and a nickel electroplating layer was formed on a copperelectrolessplating layer. Subsequently, a gold electroplating layer 0.01mm thick was formed on the nickel electroplating layer, using an aqueoussolution of the composition of 2.5 g/l of gold, 12.0 g/l of sodiumcyanide and 15.0 g/l of sodium phosphate and under the conditions of acurrent density of 0.5 Amp./dm a voltage of 2 V and a temperature of thesolution of 65C.

Using this metal mold, a low-expansion foaming resin wasinjection-molded at a cylinder temperature of 210C, under an injectionpressure of 900 kg/cm and at a metal-mold temperature of 50C, saidlowexpansion foaming resin being made up such that, to a polystyrolresin known as DlAREX l-lT-88A (refer to Example 1), 3 percent by weightof benzene-sulfonyl hydrazide as a foaming agent and Dai-Nippon-Ink-Dairen Color SP 705 (refer to Example 3) as a pigment were respectivelymixed.

On one side of a molded disc obtained, a heart pattern of a dark groundcolor was distinctly embossed.

EXAMPLE 5 A metal mold was prepared in which, in the metal mold ofExample 4, in place of the molded article of theacrylonitrile-butadiene-styrol resin a molded article of aphenolformaldehyde resin was secured to the heartshaped concave portion,and in place of the gold plating layer, a silver plating layer wasformed.

Treatment was carried out using an aqueous solution for a silver platingsolution and which had the composition of 5 g/l of silver cyanide, 4.0g/l of silver nitrate, 18.5 g/l of potassium cyanide, 13 g/l of freepotassium cyanide and 15 g/l of potassium carbonate, and under theconditions of a current density of 0.2 Amp./dm a voltage of 3 V and asolution temperature of the normal temperature. Thus, a silverelectroplating layer 0.015 mm thick was formed on the nickelelectroplating layer.

With the above metal mold, the injection molding as performed at acylinder temperature of 230C, under an injection pressure of 1,000 kg/cmand at a metalmold temperature of 50C by applying a low-expansionfoaming resin which was made up such that 1 percent by weight of diazocarbon amide was mixed as a foaming agent to anacrylonitrile-butadiene-styrol resin known under the tradename ofSUMITOMO- KRALASTIC MH (produced by Sumitomo Chemical Co., Ltd.).

On a molded article obtained, there was observed a lustrous heart-shapedpattern as in Example 4.

EXAMPLE 6 In order to represent characters of the ground color of aplastic material on a front decoration panel (dimensions: mm X 350 mm)of a cabinet for a television receiving set, a metal mold wasmanufactured as follows;

In part of the inside of a cavity of a metal mold of steel for thedecoration panel, characters were carved to a depth of 0.6 mm. Apolyethylene terephthalate resin was filled into a concave portion thusformed. The surface of the filled resin was shaved to become coplanarwith the inside, and the exposed surface was polished. Over the entirearea of the cavity inside including the resin layer, a copperelectrolessplating layer and a nickel electroplating layer weresuccessively deposited in accordance with the process of Example 1. Atin electroplating layer was formed on the nickel plating layer, usingan aqueous solution of the composition of 101 g/l of sodium stannate and10.6 g/l of caustic soda and under the conditions of a current densityof 10 Amp./dm a voltage of 20 V and a temperature of the solution of60C. In this metal mold, the shapes of the characters wereindistinguishable in the external appearance, and the whole inside ofthe cavity looked like a uniform plating surface.

When, however, the low-expansion foaming resin employed in Example 4 wasinjection-molded using the metal mold and under the conditions ofExample 4, the characters presented a lustrous ground color of the resinin a whitish foamed pattern and could be clearly read in the moldeddecoration panel.

What we claim is:

1. In a metal mold for use in injection molding of plastics material,said mold being formed of a base metal as a cavity portion and a coreportion engagable with each other to define by their inner surfaces amolding void portion, the improvement which comprises an inner surfacesection formed of a resin having a thermal conductivity lower than thatof said base metal, said section having the surface thereof that facessaid molding void portion plated with metal, said surface plated withmetal forming at least a part of the molding surface of said metal mold.

2. A metal mold as claimed in claim 1 wherein said resin is selectedfrom the group consisting of epoxy resin, polyethylene terephthalateresin, acrylonitrilebutadiene-styrene resin and phenol-formaldehyde resin.

1. In a metal mold for use in injection molding of plastics material,said mold being formed of a base metal as a cavity portion and a coreportion engagable with each other to define by their inner surfaces amolding void portion, the improvement which comprises an inner surfacesection formed of a resin having a thermal conductivity lower than thatof said base metal, said section having the surface thereof that facessaid molding void portion plated with metal, said surface plated withmetal forming at least a part of the molding surface of said metal mold.2. A metal mold as claimed in claim 1 wherein said resin is selectedfrom the group consisting of epoxy resin, polyethylene terephthalateresin, acrylonitrile-butadiene-styrene resin and phenol-formaldehyderesin.